An Experimental and Computational Assessment of Acid-Catalyzed Azide-Nitrile Cycloadditions.

MedLine Citation:

PMID:
23126486
Owner:
NLM
Status:
Publisher

Abstract/OtherAbstract:

The mechanism of the azide-nitrile cycloaddition mediated by different Bronsted and Lewis acids has been addressed through DFT calculations. In all cases activation of the nitrile substrate by the Bronsted or Lewis acid catalyst was found to be responsible for the rate enhancement. According to DFT calculations the cycloaddition proceeds in a step-wise fashion involving the initial formation of an open-chain imidoyl azide intermediate. Kinetic experiments performed using N-methyl-2-pyrrolidone as solvent and sodium azide as azide source demonstrate that all evaluated Brønsted acids have the same efficiency toward cycloaddition with benzonitrile, suggesting that hydrazoic acid is the actual dominant catalytic species in these tetrazole syntheses. Lewis acids such as Zn or Al salts perform in a similar manner activating the nitrile moiety and leading to an open-chain intermediate which subsequently cyclizes to produce the tetrazole nucleus. The most efficient catalyst evaluated was 5-azido-1-methyl-3,4-dihydro-2H-pyrrolium azide which can readily be generated in situ from aluminum chloride, sodium azide in N-methyl-2-pyrrolidone. The efficiency of this catalyst has been examined by preparation of a series of 5-substituted-1H-tetrazoles. The desired tetrazole structures were obtained in high yields within 3-10 min employing controlled microwave heating.